function Node( data ) {
  this.data = data;
  this.previous = null;
  this.next = null;
}

class DoublyLinkedList {
  constructor() {
    this.head = null;
    this.tail = null;
    this.numberOfValues = 0;
  }

  add( data ) {
    const node = new Node( data );
    if ( !this.head ) {
      this.head = node;
      this.tail = node;
    } else {
      node.previous = this.tail;
      this.tail.next = node;
      this.tail = node;
    }
    this.numberOfValues++;
  }

  remove( data ) {
    let current = this.head;
    while ( current ) {
      if ( current.data === data ) {
        if ( current === this.head && current === this.tail ) {
          this.head = null;
          this.tail = null;
        } else if ( current === this.head ) {
          this.head = this.head.next;
          this.head.previous = null;
        } else if ( current === this.tail ) {
          this.tail = this.tail.previous;
          this.tail.next = null;
        } else {
          current.previous.next = current.next;
          current.next.previous = current.previous;
        }
        this.numberOfValues--;
      }
      current = current.next;
    }
  }

  insertAfter( data, toNodeData ) {
    let current = this.head;
    while ( current ) {
      if ( current.data === toNodeData ) {
        const node = new Node( data );
        if ( current === this.tail ) {
          this.add( data );
        } else {
          current.next.previous = node;
          node.previous = current;
          node.next = current.next;
          current.next = node;
          this.numberOfValues++;
        }
      }
      current = current.next;
    }
  }

  traverse( fn ) {
    let current = this.head;
    while ( current ) {
      if ( fn ) {
        fn( current );
      }
      current = current.next;
    }
  }

  traverseReverse( fn ) {
    let current = this.tail;
    while ( current ) {
      if ( fn ) {
        fn( current );
      }
      current = current.previous;
    }
  }

  length() {
    return this.numberOfValues;
  }

  print() {
    let string = '';
    let current = this.head;
    while ( current ) {
      string += `${current.data} `;
      current = current.next;
    }
    console.log( string.trim() );
  }
}

const doublyLinkedList = new DoublyLinkedList();
doublyLinkedList.print(); // => ''
doublyLinkedList.add( 1 );
doublyLinkedList.add( 2 );
doublyLinkedList.add( 3 );
doublyLinkedList.add( 4 );
doublyLinkedList.print(); // => 1 2 3 4
console.log( 'length is 4:', doublyLinkedList.length() ); // => 4
doublyLinkedList.remove( 3 ); // remove value
doublyLinkedList.print(); // => 1 2 4
doublyLinkedList.remove( 9 ); // remove non existing value
doublyLinkedList.print(); // => 1 2 4
doublyLinkedList.remove( 1 ); // remove head
doublyLinkedList.print(); // => 2 4
doublyLinkedList.remove( 4 ); // remove tail
doublyLinkedList.print(); // => 2
console.log( 'length is 1:', doublyLinkedList.length() ); // => 1
doublyLinkedList.remove( 2 ); // remove tail, the list should be empty
doublyLinkedList.print(); // => ''
console.log( 'length is 0:', doublyLinkedList.length() ); // => 0
doublyLinkedList.add( 2 );
doublyLinkedList.add( 6 );
doublyLinkedList.print(); // => 2 6
doublyLinkedList.insertAfter( 3, 2 );
doublyLinkedList.print(); // => 2 3 6
doublyLinkedList.traverseReverse( node => {
  console.log( node.data );
} );
doublyLinkedList.insertAfter( 4, 3 );
doublyLinkedList.print(); // => 2 3 4 6
doublyLinkedList.insertAfter( 5, 9 ); // insertAfter a non existing node
doublyLinkedList.print(); // => 2 3 4 6
doublyLinkedList.insertAfter( 5, 4 );
doublyLinkedList.insertAfter( 7, 6 ); // insertAfter the tail
doublyLinkedList.print(); // => 2 3 4 5 6 7
doublyLinkedList.add( 8 ); // add node with normal method
doublyLinkedList.print(); // => 2 3 4 5 6 7 8
console.log( 'length is 7:', doublyLinkedList.length() ); // => 7
doublyLinkedList.traverse( node => {
  node.data = node.data + 10;
} );
doublyLinkedList.print(); // => 12 13 14 15 16 17 18
doublyLinkedList.traverse( node => {
  console.log( node.data );
} ); // => 12 13 14 15 16 17 18
console.log( 'length is 7:', doublyLinkedList.length() ); // => 7
doublyLinkedList.traverseReverse( node => {
  console.log( node.data );
} ); // => 18 17 16 15 14 13 12
doublyLinkedList.print(); // => 12 13 14 15 16 17 18
console.log( 'length is 7:', doublyLinkedList.length() ); // => 7
